The present invention relates to a semiconductor laser deice and to a method for fabricating the same. More particularly, it relates to an increase in the output of the semiconductor laser device.
Recent years have seen rapid widespread use of DVD (Digital Versatile Disk) devices in the fields of AV (Audio-Video) equipment, PCs (Personal Computers), and the like. In particular, great expectations have been placed on the use of recordable DVD devices (such as DVD-RAM and DVD-R) as large-capacity memory devices embedded in PCs and the like and as post-VTR (Video Tape Recorder) devices.
As pickup light sources for the foregoing DVD devices, red semiconductor lasers at wavelengths in the 650 nm band have been used. With the recent increases in the density and capacity of an optical disk, a pick-up light source capable of performing a particularly high output operation over 80 mw has been in growing demand to allow a higher-speed write operation with respect to the optical disk.
If a semiconductor laser device is increased in output, however, each of the laser facets of semiconductor laser device suffers catastrophic optical damage (hereinafter referred to as COD). The catastrophic optical damage is a degradation phenomenon caused by heat resulting from the absorption of a laser beam in the vicinity of the laser facet of the semiconductor laser device. The resulting heat degrades the portion of a semiconductor layer located in the vicinity of the laser facet. Specifically, the heat reduces the band gap of the portion of the semiconductor layer located in the vicinity of the laser facet and increases the absorption coefficient of the portion of the semiconductor layer located in the vicinity of the laser facet. Consequently, the laser beam is further absorbed in the vicinity of the laser facet.
It has been known that, in preventing COD, preliminary provision of a semiconductor layer having a large band gap and transparent to a laser beam emitted from the semiconductor laser device in a region located in the vicinity of each of the laser facets of the semiconductor laser device, i.e., the formation of a so-called window structure is effective. In particular, the formation of the window structure in a semiconductor laser device outputting a red laser beam exceeding 50 mW is inevitable to ensure the reliability of the semiconductor laser device in use.
Thus far, various methods have been proposed each for fabricating a semiconductor laser device having a window structure. One of the methods uses a phenomenon in which diffused Zn alloys a superlattice in an active layer. For example, Japanese Unexamined Patent Publication No. HEI 11-284280 discloses a method in which a window structure is formed by further forming a group III-V compound semiconductor layer containing Zn at a high concentration (hereinafter referred to as a Zn supply layer) over a region located in the vicinity of each of the laser facets of a semiconductor laser device, causing solid-phase diffusion of Zn from the Zn supply layer, and thereby disordering the active layer in the laser facet region. A method of using ZnO as a Zn diffusion source instead of the Zn supply layer is also disclosed in, e.g., Japanese Unexamined Patent Publication No. HEI 10-290043.
FIG. 10 is a perspective view showing a structure of a conventional semiconductor laser device.
As shown in FIG. 10, a conventional semiconductor laser device 70 has a structure (so-called window structure) comprising laser facet regions 713 and an internal region 712.
The internal region 712 has a multilayer structure composed of: an n-type clad layer 701 made of n-type AlGaInP; a guide layer 702a (with a thickness of 30 nm) made of AlGaInP; an active layer 702 made of a quantum well consisting of a plurality of GaInP layers and a plurality of AlGaInP layers; a guide layer 702b (with a thickness of 30 nm) made of AlGaInP; a first p-type clad layer 703 made of p-type AlGaInP containing Zn as a dopant; a current block layer 704 made of n-type AlGaInP; a second p-type clad layer 705 made of p-type AlGaInP containing Zn as a dopant; and a contact layer 706 made of p-type GaAs containing Zn as a dopant, which are stacked successively on a substrate 700 made of n-type GaAs.
The active layer 702 is composed of a repetition of the structure in which the GaInP layers are sandwiched between the AlGaInP layers.
Each of the laser facet regions 713 has a multilayer structure composed of: the n-type clad layer 701 made of n-type AlGaInP; the guide layer 702a (with a thickness of 30 nm) made of AlGaInP; an alloyed active layer 711 made of alloyed GaInP and AlGaInP; the guide layer 702b (with a thickness of 30 nm) made of AlGaInP; the first p-type clad layer 703 made of p-type AlGaInP containing Zn as a dopant; the current block layer 704 made of n-type AlGaInP; the second p-type clad layer 705 made of p-type AlGaInP containing Zn as a dopant; and the contact layer 706 made of p-type GaAs containing Zn as a dopant, which are stacked successively on the substrate 700 made of n-type GaAs.
An n-side electrode 708 made of a metal (such as an alloy of Au, Ge, or Ni) making an ohmic contact with the n-type GaAs substrate 700 is formed on the lower surface of the n-type GaAs substrate 700. A p-side electrode 709 made of a metal (such as an alloy of Cr, Pt, or Au) making an ohmic contact with the contact layer 706 is formed on the upper surface of the contact layer 706.
The alloyed active layer 711 has been disordered through the solid-phase diffusion of Zn. This increases the band gap of the alloyed active layer 711 and forms a window structure which is transparent to a laser beam emitted from the semiconductor laser device 70.
The formation of the window structure through the diffusion of Zn mentioned above increases the reliability of a semiconductor laser device and provides a semiconductor laser device capable of producing a 50-mW class output.
In either of the cases where the methods disclosed in the foregoing publications are used, thermal treatment should be performed in the steps of causing solid-phase diffusion of Zn from the diffusion source to the active layer and alloying the active layer.
If the thermal treatment is performed in the step of causing solid-phase diffusion of Zn, Zn that has been introduced as a dopant in the first p-type clad layer 703, the second p-type clad layer 705, and the contact layer 706 is diffused not only into the portions of the active layer 702 located in the laser facet regions 713 of the semiconductor laser device 70 but also into the portion of the active layer 702 located in the internal region 712 thereof. If a dopant such as Zn is diffused into the active layer 702, a nonradiative recombination center may be formed within the active layer 702 to degrade the characteristics of the semiconductor laser device 70. Otherwise, a crystal defect may be formed within the active layer 702 to reduce the lifespan of the semiconductor laser 70.
The amount of the dopant diffused into the individual semiconductor lasers composing the semiconductor laser device 70 is larger as the dopant concentrations of the first p-type clad layer 703, the second p-type clad layer 705, and the contact layer 706 are higher. As the concentration of Zn as the dopant is higher, the problems of the degraded characteristics, reduced lifespan, and the like of the semiconductor laser device accordingly become more conspicuous. To suppress the diffusion of Zn into the portion of the active layer 702 located in the internal region 712, therefore, the doping concentrations of Zn in the first p-type clad layer 703, the second p-type clad layer 705, and the contact layer 706 are preferably lowered.
However, the doping concentrations of Zn in the first p-type clad layer 703, the second p-type clad layer 705, and the contact layer 706 greatly affect the temperature characteristic of the semiconductor laser device 70. If the doping concentrations of Zn in the first p-type clad layer 703, the second p-type clad layer 705, and the contact layer 706 are lowered as described above, the band offset of the conduction band is reduced between the first p-type clad layer 703 and the active layer 702. This indicates that a sufficiently large band barrier against electrons in the conduction band cannot be formed between the first p-type clad layer 703 and the active layer 702. As a result, electrons overflowing from the active layer 702 to the first p-type clad layer 703 are increased. Even if an injected current is increased, an increase in current component contributing to light emission is reduced and a light output is saturated. The problem is encountered particularly at a high temperature that a high output cannot be produced.
FIG. 11 shows a measurement profile obtained as a result of secondary ion mass spectroscopy (hereinafter referred to as SIMS) performed with respect to the laser facet regions 713 and internal region 712 of the conventional semiconductor laser device 70 having the first p-type clad layer 703 doped with Zn at a high concentration. It is to be noted that the dopant had not been introduced into the active layer 702.
As shown in FIG. 11, Zn was mixed in the portion of the active layer 702 located in the internal region 712 irrespective of the fact the dopant had not been introduced therein intentionally. This is because Zn introduced at a high concentration into the first p-type clad layer 703 was diffused in the thermal treatment step for forming the window structure. Similar dopant diffusion also occurs during the operation of the semiconductor laser device.